1. The Field of the Invention
The present invention relates to fabrication of semiconductor structures. More particularly, the present invention relates to chip packaging processes and pre-packaging wafer preparation including wafer thinning and die separation.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. In the context of this document, the term "semiconductive substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term substrate refers to any supporting structure including but not limited to the semiconductive substrates described above. The term semiconductor substrate is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire.
In the microelectronics industry, the process of miniaturization entails the shrinking of individual semiconductor devices and crowding more semiconductor devices into a given unit area. Included in the process of miniaturization is the effort to shrink the size of chip or die packages. In the fabrication sequence, chip packaging follows the fabrication of chips or dies upon a semiconductor substrate or wafer.
After a semiconductor wafer has been fabricated and the circuits thereon have been processed to completion, the die or chip packaging process begins. The purpose of the die or chip packaging process is to place individual die into a package which can then be inserted into a printed circuit board or other substrate so as to connect the die to a larger functional circuit.
Prior to chip packaging, other steps may be needed to be undertaken in order to prepare a wafer. One step is the reducing of the thickness of a wafer. It is desirable to reduce the thickness of a wafer because a greater amount of time and expense is required to saw through a thick wafer in order to separate the dies the thereon. Typically, wafer sawing produces a precise die edge. Nevertheless, sawing adds expense, processing time, and requires expensive machinery.
It may also be desirable to thin the wafer if contaminants have entered into the backside of a wafer opposite its circuit side where the electrical circuitry has been formed. For instance, dopants may have entered the backside of the wafer during a fabrication process. These dopants will form electrical junctions that may interfere with the circuitry on the front side of the wafer. Thus, in order for the electrical circuits to properly operate, the thinning of the contaminated portion of the backside of the wafer may be required.
Conventionally, thinning of the wafer is performed prior to separating the dies from the wafer. This thinning step typically reduces the wafers to a thickness between 0.762 millimeters to about 0.2 millimeters. Several processes are available to perform the thinning operation. Specifically, a mechanical or chemical-mechanical operation, such as planarization, can be used to thin the wafers. Also, the backside of the wafer can be chemically etched in order to reduce the thickness thereof.
The wafer thinning operation can cause scratching of the top side of the wafer or the inducement of stress during the abrading operation which may cause the wafer to break. In order to perform the thinning operation, the circuit side of the wafer is placed face down upon a surface. Preferably, the circuit side of the wafer will be protected from scratching or other surface defect. A material removal operation then begins to remove material from the backside of the wafer.
Where material is moved from the backside of the wafer using a chemical etchant, it is also necessary to protect the circuit side of the wafer. Such a method includes the forming of a photoresist layer on the circuit side of the wafer. Sheets composed of a polymer material having an adhesive back can also be fitted over the circuit side of the wafer to protect the same.
It is desirable to thin wafers before packaging in order to reduce the cost of packaging the dies after separation. The separation process becomes expensive as the wafer thickness goes up. Particularly, a deeper die attach cavity is required if a wafer is thicker. As such, the combination of a deeper die attach cavity and the thicker die results in a more expensive chip package. Thus, wafer thinning is an important part of reducing the cost of chip packaging.
FIG. 1 depicts a grinding table 12 having an adhesive film 14 thereon. A semiconductor substrate 10 is on adhesive film 14. Semiconductor substrate 10 includes a die side 16 and a base layer 18. Base layer 18 has a back surface 20 thereon. Die side 16 has a plurality of die formed therein which are to be singulated by a division of semiconductor substrate 10 into a plurality of pieces. Back surface 20 is subjected to a back grinding process. The purpose of the back grinding process to be performed upon back surface 20 is to thin base layer 18 prior to singulating die side 16. As seen in FIG. 1, a distance 25 indicates a distance between a center of semiconductor substrate 10 and a grinding force 26 applied to back surface 20 by a grinding wheel 24 via a grinding pad 22 thereon. With the increase in distance 25 and/or an increase in grinding force 26, the torque product of distance 25 and grinding force 26 increases. With the increase in torque, the propensity of semiconductor substrate 10 to crack or break improperly also increases. As such, it would be desirable to reduce the propensity of semiconductor substrate 10 to break during a substrate thinning process.
After wafer thinning, the wafer is divided. Conventional techniques for die separation involves sawing and scribing processes. The sawing process uses a saw and a table to cut scribe or saw lines in the circuit side of the wafer. The wafer is placed upon the table and a rotating saw blade is brought down in contact with the circuit side of the wafer. As each scribe or saw line is cut into the wafer, a stress line forms along the crystalline interior of the wafer substantially perpendicular to the backside of the wafer. After the scribe or saw lines are cut into the wafer, a stress is applied to the scribe lines to separate the wafer and individual die. This stress may be applied via a roller or other pressure technique. Alternatively, the rotating saw blade can cut all the way through the wafer to separate the wafer and individual die.
An alternative technique to sawing the wafer to singulated dies is a scribing technique which cuts a scratch along scribe lines on the circuit side of the wafer by application of a force from a diamond-tip scribe. As in sawing, the dies are separated by applying a stress to the wafer, such as a roller applied to a surface of the wafer. Upon the application of the pressure from the roller, individual dies will be separated as they break away from the consolidated wafer along the scratched scribe lines. Due to the crystalline structure of the wafer, the separation of the die will follow the scribe line approximately perpendicular to the opposing surfaces of the wafer. As such, stress will cause the wafer to break along the scratched scribe lines.
FIG. 2 depicts semiconductor substrate 10 including die side 16 and base layer 18. Semiconductor substrate 10 has saw or scribe lines marked within die side 16 and above stress lines 29. Each scribe line is cut into die side 16 by a cutting tool 28 with a cutting force 30. Cutting tool 28 can be a diamond tipped scribe or a rotating saw blade. Once the saw or scribe lines are cut within die side 16, a roller 32 having a surface 34 applies a roller force 36 to die side 16 to separate a singulated die 19 along stress line 29.
While it is desirable to thin a wafer prior to singulating the dies thereon due to the lower cost of packaging and the shorter time of throughput, thinning the wafer also causes an increased likelihood of breaking the wafer prematurely and prior to singulation. Breaking the wafer prematurely can occur during any of a chemical, mechanical, or chemical-mechanical thinning operation, wherein forces are induced within the wafer. This problem is further compounded by a desire to fabricate more dies upon a semiconductor wafer. In order to put more dies on a semiconductor wafer, the diameter of a semiconductor wafer is increased. With an increase in diameter, an increase in stress is realized as pressure is applied to the wafer during scribing or sawing operations. As the radius of the pressure from the center of the wafer increases, the torque product also increases and the propensity of the larger wafer to break goes up. A warped or cracked wafer reduces yield and causes other problems in the subsequent chip packaging process.
Given the foregoing, it would be advantageous to reduce the forces, including stress-induced forces, in the wafer during the wafer thinning process. It would also be desirable to accomplish a technique of thinning the wafer prior to packaging individual die while decreasing the propensity of the wafer to break. It would also be advantageous to develop such a technique for use with larger wafers.